In a method for mounting various parts on a workpiece with tapping screws, the tapping screws are screwed into prepared holes formed in the workpiece while tapping female threads, so that the parts are supported between the workpiece and the heads of the tapping screws. Such a mounting method is generally employed for, for example, mounting small parts of an electronic device on a workpiece. When the mounting method is employed, attention must be paid to the treatment of powdery chips (i.e., forming chips) generated during screwing of a tapping screw into a prepared hole of a workpiece. In the mounting method, when a tapping screw is screwed into a prepared hole of a workpiece, as the tapping screw enters the prepared hole, a female thread is gradually formed at the inner wall of the prepared hole, whereby shavings are continuously generated through formation of the female thread. The shavings are scraping of the material of the workpiece. The thus-generated shavings may be scattered and deposited on a part to be mounted or on the workpiece. The shavings deposited on the part or the workpiece may adversely affect other parts mounted on the workpiece. When, for example, an electronic circuit or a similar part is mounted on the workpiece, scattering of chips generated during formation of a female thread (hereinafter the chips may be referred to as “female-thread-forming chips”) may cause short-circuit of the electronic circuit, resulting in, for example, breakage or burnout of devices.
In order to solve such a problem, scattering of female-thread-forming chips, which are continuously generated during screwing of a tapping screw into a prepared hole of a workpiece, must be prevented through specific means, thereby preventing deposition of the female-thread-forming chips on parts or the workpiece. A tapping screw having means for preventing deposition of female-thread-forming chips on parts or a workpiece has been proposed and disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 2002-70824 (Patent Document 1), No. 2002-257120 (Patent Document 2), or No. 2002-257121 (Patent Document 3).
The tapping screws proposed in the aforementioned patent documents include a male threaded portion (i.e., shank) having, on the periphery thereof, a coating layer. The coating layer is formed through application of a resin adhesive agent containing microcapsules encapsulating an epoxy adhesive, or application of a coating material containing microcapsules encapsulating an absorbent/adsorbent having a viscosity of 40 to 100 cP as a primary component. However, none of these patent documents describes an effective component for forming such a coating layer; i.e., base components constituting the coating layer are not disclosed in detail.
In a tapping screw having thereon a coating layer formed from a resin adhesive agent whose components are partially specified, the coating layer is formed from a resin adhesive agent containing densely dispersed microcapsules encapsulating an epoxy adhesive. When the tapping screw is screwed into a prepared hole of a workpiece, the microcapsules are broken by pressure applied thereto during formation of a female thread, and the coating layer may exhibit an adhesion effect on female-thread-forming chips. Thus, by virtue of its adhesion effect exerted under application of pressure during formation of the female thread, the coating layer may hold female-thread-forming chips generated during screwing of the tapping screw; i.e., the coating layer may function so as to prevent scattering of female-thread-forming chips on parts or the workpiece.
However, as described above, none of the aforementioned patent documents discloses components of a coating layer (in particular, an effective component for forming the coating layer) formed on the periphery of a male threaded portion (i.e., shank) of a tapping screw. Therefore, it is absolutely unclear whether or not the coating layer sufficiently exhibits the aforementioned function of capturing female-thread-forming chips when the tapping screw is screwed into a prepared hole of a workpiece. In addition, the coating layer is required to have the following characteristics for exhibiting the function of capturing female-thread-forming chips.
Firstly, the coating layer is required to have a characteristic such that it is strongly bonded to the periphery of the shank of the tapping screw and is never removed therefrom until completion of screwing of the tapping screw into the prepared hole of the workpiece. Secondly, the coating layer is required to have a characteristic such that it expresses its potential adhesive function at a desired timing by means of pressure applied thereto during gradual formation of a female thread until completion of screwing of the tapping screw into the prepared hole of the workpiece. Thirdly, the coating layer is required to have a characteristic such that it exhibits a lubricating function to such an extent that the tapping screw can be smoothly screwed into the prepared hole of the workpiece under a predetermined operation load. Regarding these characteristics, the coating layer formed from the aforementioned resin adhesive agent containing densely dispersed microcapsules encapsulating an epoxy adhesive poses problems described below.
Since components of such a coating agent for forming the coating layer are not disclosed in detail, the adhesion strength of the coating layer to the shank of the tapping screw cannot be evaluated. However, the adhesion strength of the coating layer is not considered in the aforementioned patent documents. Thus the coating layer cannot be regarded as having the aforementioned first characteristic.
It is asserted that the coating agent for forming the coating layer is a resin adhesive agent containing densely dispersed microcapsules encapsulating an epoxy adhesive. On the basis of this assertion, the coating agent is thought to be prepared by, for example, dispersing numerous epoxy-adhesive-containing microcapsules in a resin emulsion as an effective coating component. In the coating agent, the size of resin particles of the resin emulsion which is an effective coating component is as small as about 0.01 μm, whereas the size of microcapsules is very large (i.e., 1 μm or more). Therefore, the coating agent poses a problem in that a large number of microcapsules cannot be uniformly dispersed in the resultant coating layer; i.e., the microcapsules are not uniformly distributed over the surface of a metallic member to which the coating layer is bonded.
In view of the relationship between the microcapsule size and the size of resin particles of the emulsion, the thickness of a coating layer formed must be increased. However, when a thick coating layer is formed on a tapping screw, the coating layer is likely to separate from the screw in an early stage of screwing of the screw into a prepared hole of a workpiece; i.e., the coating layer cannot maintain strong adhesion to the screw until completion of screwing of the screw into the prepared hole of the workpiece.
In addition, when such a thick coating layer is formed on a tapping screw, progress of breakage of microcapsules contained in the coating layer does not keep up with the rate of screwing of the tapping screw in a prepared hole of a workpiece (i.e., the rate of formation of a female thread). Thus, the coating layer cannot achieve expression of its potential adhesive function at a desired timing by means of pressure applied thereto upon formation of a female thread until completion of screwing of the tapping screw into the prepared hole of the workpiece. That is, since the coating layer does not have the aforementioned second characteristic, the coating layer cannot exhibit the function of capturing female-thread-forming chips (chips produced during formation of a female thread) at a desired timing, and scattering of the chips is inevitable.
Also, use of a lubricant in the aforementioned coating agent, whose components are not described in detail, is not disclosed. In addition, no attention is paid to the lubricating function required for a coating layer between the male threaded portion of a tapping screw and the inner wall of a prepared hole of a workpiece during formation of a female thread. Therefore, the coating layer cannot be regarded as having the aforementioned third characteristic.
In view of the foregoing, a main object of the present invention is to provide a coating agent capable of forming a coating layer which has at least the aforementioned three characteristics, and which functions so as to prevent scattering of female-thread-forming chips generated during formation of a female thread through screwing of a tapping screw into a prepared hole of a workpiece. Another object of the present invention is to provide a method for forming the coating layer on the periphery of a tapping screw by use of the coating agent. Yet another object of the present invention is to provide a tapping screw having thereon the coating layer (hereinafter the tapping screw may be referred to as a “coated tapping screw”).